Silicon carbide fiber is used to provide mechanical strength at high temperatures to fibrous products, such as high temperature insulation, belting, gaskets, or curtains, or as reinforcements in plastic, ceramic, or metal matrices of high performance composite materials. To provide mechanical strength to these products or materials, the silicon carbide has relatively fine grain sizes and low density, one third as compared to steel. Silicon carbide is also used in advanced nuclear fuel elements to provide mechanical stability and higher thermal conductivity to the oxide based fuel. The silicon carbide can also function as a diffusion barrier to the release of fission products.
Silicon carbide exists in approximately 250 crystalline forms, including the alpha polymorph or polytype and the beta polymorph, each of which has a different crystal structure. Silicon carbide is commercially available in many forms, such as powders or particulates, fibers, whiskers, or cloth, depending on the polymorph. Alpha silicon carbide has a hexagonal crystal structure and a decomposition temperature of approximately 2730° C. Alpha silicon carbide is conventionally manufactured as a powder on a large scale for use in monolithic (non-fiber) silicon carbide products. Monolithic forms of alpha silicon carbide are known in the art, such as HEXOLOY® SA silicon carbide from Saint-Gobain Advanced Ceramics (Niagara Falls, N.Y.). HEXOLOY® SA silicon carbide exhibits reliable performance in air at temperatures greater than 1900° C. and is used at temperatures of at least 2200° C. in inert environments. Monolithic (non-fiber) forms of alpha silicon carbide contain no alpha silicon carbide fibers or beta silicon carbide fibers. The HEXOLOY® SA silicon carbide is produced by pressureless sintering of silicon carbide powder. One known process of forming monolithic alpha silicon carbide is to heat sub-micron alpha silicon carbide powder formed by the Acheson process to a temperature of 2200° C. and pressureless sinter the alpha silicon carbide powder into a product shape using sintering aids, such as boron or calcium. Another process for forming monolithic alpha silicon carbide is the Lely process, in which silicon carbide powder is sublimated in an argon atmosphere at a temperature of 2500° C. and re-deposited into single crystals. Monolithic alpha silicon carbide may also be formed by reaction bonding silicon and carbon powder, or by chemical vapor deposition (CVD) using gases such as silane (SiH4), propane (C3H8), or more complex gases to form a coating.
Beta silicon carbide has a cubic or zinc blende crystal structure. The silicon carbide composite industry is based on the use of beta silicon carbide fibers in a beta silicon carbide matrix. The crystalline structure of the matrix is the same as the crystalline structure of the fibers to maintain phase stability at elevated temperatures. Beta silicon carbide fibers are commercially available, such as SYLRAMIC® silicon carbide fibers from COI Ceramics, Inc. (San Diego, Calif.), HI-NICALON™ ceramic fibers and HI-NICALON™ type S ceramic fibers from Nippon Carbon (Tokyo, Japan) and distributed through COI Ceramics, Inc (San Diego, Calif.), and TYRANNO FIBER® from Ube Industries, Ltd. (Tokyo, Japan). Beta silicon carbide fibers are used with the beta silicon carbide matrix in ceramic matrix composites (CMCs). However, CMCs including the beta silicon carbide matrix and beta silicon carbide fibers have limited temperature use due to fiber degradation. These CMCs may be used for short times at a temperature up to 1400° C. or may be used continuously at a temperature below 1200° C. Beta silicon carbide particulate powder is conventionally produced by the Acheson process in which silicon dioxide and carbon are reacted in an electric resistor furnace at a temperature between 1600° C. and 2500° C. The beta silicon carbide may also be formed by the conversion of silicon monoxide and carbon into fibers of beta silicon carbide. The commercially available fibers of beta silicon carbide are produced using a pre-ceramic polymer conversion route enabling extrusion of continuous fibers, followed by high temperature sintering in a controlled atmosphere to about 1600° C. Other processes of producing beta silicon carbide require activation of the carbon using oxygen to enhance silicon dioxide or silicon monoxide reaction with the carbon. Moreover, attempts to make alpha silicon carbide fibers by these processes have not been successful as further heat treatment of the formed beta silicon carbide fibers causes the fibers to lose thermal and mechanical properties at temperatures above 1700° C., and heating to effect crystalline conversion (from beta to alpha) results in significant mechanical property degradation.
It would be desirable to produce fibers of alpha silicon carbide for use in a variety of high temperature and composite applications. It would also be desirable to produce the alpha silicon carbide fibers in an economical manner.